Processing of the 2S storage protein pronapin in Brassica napus and in transformed tobacco

Structural Characterization of the Allergenic 2S Albumin Cor a 14: Comparing Proteoform Patterns across Hazelnut Cultivars

The hazelnut allergen Cor a 14 belongs to the 2S albumins, a family of heterodimeric seed storage proteins exhibiting a high degree of structural diversity. Given its relevance as an allergen and the potential to elicit severe reactions, elucidation of the sequence heterogeneity of naturally occurring Cor a 14 is essential for the development of reliable diagnostics and risk evaluation. We therefore performed a comprehensive survey on the proteoforms of Cor a 14 and determined their quantitative distribution in three different hazelnut cultivars by a combinatory HPLC-HRMS approach including bottom-up and intact mass analysis. Compared with the Cor a 14 prototype sequence, we identified three sequence polymorphisms, two of the small and one of the large subunit, and elucidated their specific pairing on the protein level. Furthermore, we located a pronounced microheterogeneity on the protein termini and, for the first time, provide data on varying proteoform patterns between different cultivars of an allergenic seed. Together, these data present the basis for a more detailed investigation on the allergenicity of Cor a 14 in different cultivars and constitute, to be best of our knowledge, the largest set of proteoforms so far reported for a 2S albumin.

Structural and functional characterization of recombinant napin-like protein of Momordica charantia expressed in methylotrophic yeast Pichia pastoris

Napin and napin-like proteins belong to the 2S albumin seed storage family of proteins and have been shown to display a variety of biological activities. However, due to a high degree of polymorphism, purification of a single napin or napin-like protein exhibiting biological activity is extremely difficult. In the present study, we have produced the napin-like protein of Momordica charantia using the methylotrophic Pichia pastoris expression system. The recombinant napin-like protein (rMcnapin) secreted in the extracellular culture supernatant was enriched by ammonium sulfate precipitation, and purified using size exclusion chromatography at a yield of ∼290 mg/L of culture. Secondary structure analysis of the purified rMcnapin revealed it to be predominantly α-helical with minimal β strand content. CD spectroscopic and fluorescence spectroscopic analyses revealed the rMcnapin to be stable at a wide range of temperatures and pH. The rMcnapin exhibited antifungal activity against Trichoderma viride with an IC50 of ∼3.7 μg/ml and trypsin inhibitor activity with an IC50 of 4.2 μM. Thus, large amounts of homogenous preparations of the biologically active rMcnapin could be obtained at shake flask level, which is otherwise difficult from its natural source.

A strategy for targeting recombinant proteins to protein storage vacuoles by fusion to Brassica napus napin in napin-depleted seeds

Seeds are capable of accumulating high levels of seed storage proteins (SSP), as well as heterologous proteins under certain conditions. Arabidopsis thaliana was used to develop a strategy to deplete seeds of an endogenous SSP and then replenish them with the same protein fused to a heterologous protein. In several other studies, competition with endogenous SSP for space and metabolic resources was shown to affect the accumulation of recombinant proteins in seeds. We used RNAi to reduce the expression of the five napin genes and deplete the seeds of this SSP. Targeting a recombinant protein to a vacuole or structure within the seed where it can be protected from cytosolic proteases can also promote its accumulation. To achieve this, a synthetic Brassica napus napin gene (Bn napin) was designed that was both impervious to the A. thaliana napin (At napin) RNAi construct and permitted fusion to a heterologous protein, in this case green fluorescent protein (GFP). GFP was placed in several strategic locations within Bn napin with consideration to maintaining structure, processing sites and possible vacuolar targeting signals. In transgenic A. thaliana plants, GFP was strongly localized to the seed protein storage vacuole in all Bn napin fusion configurations tested, but not when expressed alone. This SSP depletion-replenishment strategy outlined here would be applicable to expression of recombinant proteins in industrial crops that generally have large repertoires of endogenous SSP genes.

2S Albumin Storage Proteins: What Makes them Food Allergens?

2S albumin storage proteins are becoming of increasing interest in nutritional and clinical studies as they have been reported as major food allergens in seeds of many mono- and di-cotyledonous plants. This review describes the main biochemical, structural and functional properties of these proteins thought to play a role in determining their potential allergenicity. 2S albumins are considered to sensitize directly via the gastrointestinal tract (GIT). The high stability of their intrinsic protein structure, dominated by a well-conserved skeleton of cysteine residues, to the harsh conditions present in the GIT suggests that these proteins are able to cross the gut mucosal barrier to sensitize the mucosal immune system and/or elicit an allergic response. The flexible and solvent-exposed hypervariable region of these proteins is immunodominant and has the ability to bind IgE from allergic patients sera. Several linear IgE-binding epitopes of 2S albumins spanning this region have been described to play a major role in allergenicity; the role of conformational epitopes of these proteins in food allergy is far from being understood and need to be investigated. Finally, the interaction of these proteins with other components of the food matrix might influence the absorption rates of immunologically reactive 2S albumins but also in their immune response.

In vitro refolded napin-like protein of Momordica charantia expressed in Escherichia coli displays properties of native napin

Napins belong to the family of 2S albumin seed storage proteins and are shown to possess antifungal activity. Napins, in general, consist of two subunits (derived from single precursor) linked by disulphide bridges. Usually, reducing environment of the E. coli cytosol is not conducive for proper folding of heterodimeric proteins containing disulphide bridges. Present investigation reports for the first time expression of napin-like protein of Momordica charantia (rMcnapin) in E. coli and its in vitro refolding to produce biologically active protein. Full-length cDNA encoding napin-like protein (2S albumin) was isolated from M. charantia seeds by immunoscreening a cDNA expression library. The cDNA consisted of an open reading frame encoding a protein of 140 amino acid residues. The 36 amino acids at the N-terminus represent the signal and propeptide. The region encoding small and large chains of the M. charantia napin is separated by a linker of 8 amino acid residues. The region encoding napin (along with the linker) was PCR amplified, cloned into pQE-30 expression vector and expressed in E. coli. rMcnapin expressed as inclusion bodies was solubilized and purified by Ni2+-NTA affinity chromatography. The denatured and reduced rMcnapin was refolded by rapid dilution in an alkaline buffer containing glycerol and redox couple (GSH and GSSG). Refolded His-rMcnapin displayed similar spectroscopic properties as that of mature napin-like protein of M. charantia with 48.7% alpha-helical content. In addition, it also exhibited antifungal activity against T. hamatum with IC50 of 3 microg/ml. Refolded His-rMcnapin exhibited approximately 90% antifungal activity when compared with that of mature napin-like protein of M. charantia. Thus, a heterologous expression system and in vitro refolding conditions to obtain biologically active napin-like protein of M. charantia were established.

Thermostability and in vitro digestibility of a purified major allergen 2S albumin (Ses i 1) from white sesame seeds (Sesamum indicum L.)

A major 2S albumin allergen, Ses i 1, from white sesame seeds was purified to homogeneity, characterized and identified using proteomic techniques. Ses i 1 exhibited a molecular weight of 12062 Da, although an extensive C-terminal clipping of the small subunit was observed. In addition, the N-terminal glutamine of the small subunit had been converted to pyroglutamate and a variant of the large subunit which had lost the N-terminal glutamine was also detected. The protein was thermo-stable up to 90 degrees C at neutral and acid pH, retaining its monomeric state and showing minimal alterations, which were reversible on cooling, in a predominantly alpha-helical secondary structure, as shown by circular dichroism and Fourier transform-infrared spectroscopy. Ses i 1 was also highly resistant to digestion using a physiologically relevant in vitro gastrointestinal model system. After 2 h of gastric digestion, the allergen remained completely intact and only the small subunit was cleaved during 2 h of subsequent duodenal digestion, leaving a major IgE epitope region of this protein intact. Neither prior heating of the Ses i 1 nor the presence of the physiological surfactant phosphatidylcholine affected the pattern of proteolysis. These findings are consistent with those found for the 2S albumin allergen from Brazil nut, Ber e 1, and suggest that Ses i 1 may preserve its structure from the degradation in the gastrointestinal tract, a property thought to be crucial for both a protein to sensitise the mucosal immune system and provoke an allergic reaction in a sensitised individual.

A Recombinant Precursor of the Mustard Allergen Sin a 1 Retains the Biochemical and Immunological Features of the Heterodimeric Native Protein

BACKGROUND

Mustard has been an important cause of food allergy of increasing incidence in the last years. Sin a 1, a storage 2S albumin, is the most relevant allergen from this spice.

METHODS

Pichia pastoris has been used as host for the recombinant production of the precursor form of Sin a 1 (rproSin a 1). rproSin a 1 was purified to homogeneity by three chromatographic steps: gel filtration, anion exchange and reverse-phase HPLC. Molecular characterization was performed using Edman degradation, mass spectrometry, amino acid composition, and circular dichroism. Immunological properties were analyzed by immunoblotting, ELISA, and ELISA inhibition experiments.

RESULTS

We overexpressed rproSin a 1 as a single polypeptide with both large and small chains linked by an internal processed fragment at high yield. The purified rproSin a 1 (>95%) was obtained as a monomeric and soluble protein. rproSin a 1 showed equivalent structural and immunological properties to natural heterodimeric Sin a 1 allergen. rproSin a 1 was recognized by 75% of the patients allergic to mustard. The inhibitory capacity of rproSin a 1 to the total allergenicity of mustard extracts varied from 13 to 83% in different patients, with a mean value of 54%.

CONCLUSIONS

rproSin a 1 is a good candidate to replace natural allergen in diagnosis protocols of mustard allergy. P. pastoris has been demonstrated to be a suitable expression system for the production of allergenic derivates of Sin a 1 that could be used for immunotherapy purposes in future.

Stability of the major allergen Brazil nut 2S albumin (Ber e 1) to physiologically relevant in vitro gastrointestinal digestion

The major 2S albumin allergen from Brazil nuts, Ber e 1, was subjected to gastrointestinal digestion using a physiologically relevant in vitro model system either before or after heating (100 degrees C for 20 min). Whilst the albumin was cleaved into peptides, these were held together in a much larger structure even when digested by using a simulated phase 1 (gastric) followed by a phase 2 (duodenal) digestion system. Neither prior heating of Ber e 1 nor the presence of the physiological surfactant phosphatidylcholine affected the pattern of proteolysis. After 2 h of gastric digestion, approximately 25% of the allergen remained intact, approximately 50% corresponded to a large fragment of M(r) 6400, and the remainder comprised smaller peptides. During duodenal digestion, residual intact 2S albumin disappeared quickly, but a modified form of the 'large fragment' remained, even after 2 h of digestion, with a mass of approximately 5000 Da. The 'large fragment' comprised several smaller peptides that were identified, by using different MS techniques, as deriving from the large subunit. In particular, sequences corresponding to the hypervariable region (Q37-M47) and to another peptide (P42-P69), spanning the main immunoglobulin E epitope region of 2S albumin allergens, were found to be largely intact following phase 1 (gastric) digestion. They also contained previously identified putative T-cell epitopes. These findings indicate that the characteristic conserved skeleton of cysteine residues of 2S albumin family and, particularly, the intrachain disulphide bond pattern of the large subunit, play a critical role in holding the core protein structure together even after extensive proteolysis, and the resulting structures still contain potentially active B- and T-cell epitopes.

Mass spectrometry and structural characterization of 2S albumin isoforms from Brazil nuts (Bertholletia excelsa)

Proteomic approaches have been used to characterise the main 2S albumin isoforms from Brazil nuts (Bertholletia excelsa). Whilst most isoforms ( approximately 10 discrete protein species) exhibited molecular masses of around 12 kDa with a high amino acid sequence homology, important charge heterogeneity was found, with pIs varying between 4.6 and 6.6, with one >or=7.0. Proteomic analysis showed that these corresponded to a total of six National Center for Biotechnology Information (NCBI) accessions and that three isoforms had been purified to homogeneity corresponding to gi/384327, 112754 and 99609. The latter sequence corresponds to an isoform, previously only identified at the nucleotide sequence level, had a slightly higher molecular weight (13.4 kDa), and with noticeable differences in the primary structure. Proteins corresponding to six different NCBI accessions were identified, the heterogeneity of which had been increased by posttranslational processing. Evidence was found of cyclization of the N-terminal glutamine residue in two isoforms, together with ragged C-termini, indicative of carboxypeptidase activity within the vacuole following posttranslational processing. No evidence of glycosylation was found. Circular dichroism (CD) and Fourier transform-infrared (FT-IR) spectroscopy indicated all the studied isoforms were predominantly alpha-helical in nature, but that the Mr 13400 species was structurally distinct, with a higher proportion of alpha-helical structure.

Precursor of the inactive 2S seed storage protein from the Indian mustard Brassica juncea is a novel trypsin inhibitor. Charaterization, post-translational processing studies, and transgenic expression to develop insect-resistant plants

A number of trypsin inhibitor (TI) genes have been used to generate insect-resistant plants. Here we report a novel trypsin inhibitor from Indian mustard Brassica juncea (BjTI) that is unique in being the precursor of a 2S seed storage protein. The inhibitory activity is lost upon processing. The predicted amino acid sequence of the precursor based on the B. juncea 2S albumin (Bj2S) gene cloned and sequenced in this laboratory (Bj2Sc; GenBank(TM) accession number ) showed a soybean-TI active site-like motif GPFRI at the expected processing site. The BjTI was found to be a thermostable Kunitz type TI that inhibits trypsin at a molar ratio of 1:1. The 20-kDa BjTI was purified from midmature seeds and found to be processed in vitro to 9- and 4-kDa subunits upon incubation with seed extract. The Bj2Sc sequence was expressed in Escherichia coli pET systems as the inhibitor precursor. The radiolabeled gene product was expressed in vitro in a coupled transcription-translation system and showed the expected processing into subunits. Two in vitro expressed pre-2S proteins, mutated at Gly and Asp residues, were processed normally to mature subunits, showing thereby no absolute requirement of Gly and Asp residues for processing. Finally, the 2S gene was introduced into tobacco and tomato plants. Third generation transgenics expressing BjTI at 0.28-0.83% of soluble leaf proteins showed remarkable resistance against the tobacco cutworm, Spodoptera litura. This novel TI can be used in transforming seed crops for protection to their vegetative parts and early seed stages, when insect damage is maximal; as the seeds mature, the TI will be naturally processed to the inactive storage protein that is safe for consumption.

Recombinant pronapin precursor produced in Pichia pastoris displays structural and immunologic equivalent properties to its mature product isolated from rapeseed

2S albumin storage proteins from rapeseed (Brassica napus), called napins, consist of two different polypeptide chains linked by disulphide bridges, which are derived by proteolytic cleavage from a single precursor. The precursor form of the napin BnIb (proBnIb) has been cloned using a PCR strategy and sequenced. The amino-acid sequence deduced from the clone includes 31 residues of the small chain and 75 of the large chain, which are connected by the peptide Ser-Glu-Asn. Expression of the cDNA encoding proBnIb has been carried out in the methylotrophic yeast Pichia pastoris. The induced protein was secreted to the extracellular medium at a yield of 80 mg.L(-1) of culture and was purified by means of size-exclusion chromatography and reverse phase-HPLC. Recombinant proBnIb appeared properly folded as its molecular and spectroscopic properties were equivalent to those of the mature heterodimeric protein. As 2S albumin storage proteins from Brassicaceae have been shown to be type I allergy inducers, the immunological activity of the recombinant proBnIb was analysed as a measure of its structural integrity. The immunological properties of the recombinant precursor and the natural napin were indistinguishable by immunoblotting and ELISA inhibition using polyclonal antisera and sera of patients allergic to mustard and rapeseed. In conclusion, the recombinant expression of napin precursors in P. pastoris has been shown to be a successful method for high yield production of homogeneous and properly folded proteins whose polymorphism and complex maturation process limited hitherto their availability.

Processing of preproricin in transgenic tobacco

The plant protein toxin ricin has found widespread application as a potential therapeutic agent for many human diseases and in disease-model systems such as those involving apoptosis. Genetic engineering and expression of the complete two-polypeptide chain toxin have only been possible in plants, specifically in transgenic tobacco carrying the preproricin gene under the control the cauliflower mosaic virus 35S promoter. Production of modified ricin for altered controllable activity and/or fusion therapeutics to target delivery requires knowledge of the heterologous processing that occurs when preproricin is expressed in tobacco. Here, recombinant ricin from transgenic tobacco was purified using lectin affinity chromatography and characterized using various biochemical and biophysical techniques. Coomassie blue staining of an SDS-PAGE gel of lactose-agarose purified material identified predominant proteins of 30 and 35 kDa molecular weight. Western analysis using anti-ricin a- and b-chain antibodies confirmed the expression and purification of recombinant ricin, with identical protein banding profiles to that of authentic castor-bean-derived ricin. High-resolution gel filtration chromatography characterized the lactose binding complex as a 66-kDa native molecular weight protein which could be separated into 30- and 35-kDa proteins upon incubation with the reducing agent dithiothreitol. N-terminal sequencing of the recombinant ricin a-chain revealed that an equimolar ratio of two alternately processed peptides was present, which varied by an additional amino acid derived from the signal peptide. Similar analysis of ricin b-chain again identified two forms of this polypeptide as well; however, full-length ricin b-chain and b-chain missing the first alanine residue were present at 11:1 molar ratios. Transgenic tobacco plants expressing ricin were used to develop a stable cell suspension culture system from callus induced with the growth regulators 2,4-dichlorophenoxyacetic acid and 6-benzylaminopurine. Double sandwich enzyme-linked immunosorbent assay using anti-ricin b-chain antibodies and Western analysis identified soluble ricin in the media of the cultures, indicating that cell cultures provide a safe and simple means to produce properly processed recombinant ricin.

Deposition of storage proteins

Plants store amino acids for longer periods in the form of specific storage proteins. These are deposited in seeds, in root and shoot tubers, in the wood and bark parenchyma of trees and in other vegetative organs. Storage proteins are protected against uncontrolled premature degradation by several mechanisms. The major one is to deposit the storage proteins into specialized membrane-bounded storage organelles, called protein bodies (PB). In the endosperm cells of maize and rice prolamins are sequestered into PBs which are derived from the endoplasmic reticulum (ER). Globulins, the typical storage proteins of dicotyledonous plants, and prolamins of some cereals are transported from the ER through the Golgi apparatus and then into protein storage vacuoles (PSV) which later become transformed into PBs. Sorting and targeting of storage proteins begins during their biosynthesis on membrane-bound polysomes where an N-terminal signal peptide mediates their segregation into the lumen of the ER. After cleavage of the signal peptide, the polypeptides are glycosylated and folded with the aid of chaperones. While still in the ER, disulfide bridges are formed which stabilize the structure and several polypeptides are joined to form an oligomer which has the proper conformation to be either deposited in ER-derived PB or to be further transferred to the PSV. At the trans-Golgi cisternae transport vesicles are sequestered which carry the storage proteins to the PSV. Several storage proteins are also processed after arriving in the PSVs in order to generate a conformation that is capable of final deposition. Some storage protein precursors have short N- or C-terminal targeting sequences which are detached after arrival in the PSV. Others have been shown to have internal sequence regions which could act as targeting information. In some cases positive targeting information is known to mediate sorting into the PSV whereas in other cases aggregation and membrane association seem to be major sorting mechanisms.

1H NMR assignment and global fold of napin BnIb, a representative 2S albumin seed protein

Napin BnIb is a representative member of the 2S albumin seed proteins, which consists of two polypeptide chains of 3.8 and 8.4 kDa linked by two disulfide bridges. In this work, a complete assignment of the 1H spectra of napin BnIb has been carried out by two-dimensional NMR sequence-specific methods and its secondary structure determined on the basis of spectral data. A calculation of the tertiary structure has been performed using approximately 500 distance constraints derived from unambiguously assigned NOE cross-correlations and distance geometry methods. The resulting global fold consists of five helices and a C-terminal loop arranged in a right-handed spiral. The folded protein is stabilized by two interchain disulfide bridges and two additional ones between cysteine residues in the large chain. The structure of napin BnIb represents a third example of a new and distinctive folding pattern first described for the hydrophobic protein from soybean and nonspecific lipid transfer proteins from wheat and maize. The presence of an internal cavity is not at all evident, which rules out in principle the napin BnIb as a carrier of lipids. The determined structure is compatible with activities attributed to these proteins such as phospholipid vesicle interaction, allergenicity, and calmodulin antagonism. Given the sequence homology of BnIb with other napins and napin-type 2S albumin seed proteins from different species, it is likely that all these proteins share a common architecture. The determined structure will be crucial to establish structure-function relationships and to explore the mechanisms of folding, processing, and deposition of these proteins. It will also provide a firm basis for a rational use of genetic engineering in order to develop improved transgenic plants.

Structural comparison of the precursor and the mature form of napin, the 2S storage protein in Brassica napus

The 2S storage protein napin from Brassica napus var. L. is synthesised as a precursor protein at the endoplasmic reticulum and transported along a gradient of decreasing pH to the vacuole, where two propeptides are removed to produce mature napin. The structures of pronapin expressed in insect cells and mature napin from rape seed were characterised. Limited proteolysis with several endoproteases cleaved primarily in the propeptides, suggesting that the propeptides are exposed to the exterior of the protein. Immunological comparison in parallel with circular dichroic spectrometry, both at neutral and acid pH, indicated that the propeptides had only a minor influence on the conformation of the regions of the molecule that correspond to mature napin.